The present invention relates generally to scanning projection apparatuses and more particularly relates to light beam scanning projection displays with phase detection and compensation between the timing of modulating the light beam and the position of the scanned light beam.
In a scanning projection display, a collimated light beam is deflected about two orthogonal axes to move across an image field by either one beam scanner with bi-axial scanning capability or by two single axis beam scanners with their scan axes aligned perpendicular to each other. An image field can be established by sweeping the light beam either in raster pattern or in Lissajous pattern. Images, texts or patterns can be formed over the image field by determining light beam position and modulating the power of the light beam to form desired image pixels according to the received image or video information.
In order to make a compact image projection system, micromechanical scanning mirror is typically employed as the beam scanner. Electrical drive signals such as alternating voltage or alternating current are applied to the beam scanner to generate mechanical oscillation of the mirror. The beam scanners may be operated on or near their resonant frequencies to obtain maximum mechanical gain or large scan angle in bi-directional motion with sinusoidal scan trajectories. For raster scan system, the horizontal scanning motion is typically sinusoidal at a high frequency depending on image resolution while the vertical scanning motion is a controlled sawtooth motion at 50 Hz or 60 Hz depending on image refresh rate. For Lissajous scan system, both the horizontal and the vertical scanning motions are sinusoidal with a carefully chosen frequency ratio such that a stable and repeatable scan pattern can be achieved.
In order to project a scanned image, the correct position of the deflected light beam needs to be determined such that the light beam can be modulated to form desired image pixels according to received image information. For sinusoidal scanning, one can calculate the light beam position relative to the scan trajectory based on mathematical equation of sinusoidal motion once the scan frequency and the timing of crossover, the timing when the light beam passing through the center of the scan span, are known. For micromechanical oscillator, the oscillation frequency is directly related to the frequency of the drive signal; however, the timing of crossover is related to the damping and the change of resonance frequency which depend on conditions of the operating environments such as temperature, moisture, air pressure, and etc. If the beam scanner is operated at a fixed frequency near the resonance frequency, a slight change in the resonance frequency will cause phase change between the drive signal and the oscillation motion of the beam scanner. In order to form image pixel in the correct location, the timing of modulating the light beam needs to be adjusted accordingly. The projected image pixels will become displaced and distorted if the phase difference between the timing of modulating the light beam and the position of the scanned light beam is not compensated correctly.
Furthermore, human eye safety is a primary concern in a front projection apparatus employing light beam scanning schemes because high output power of the light source is required to generate image with enough brightness. For safety precaution, it is necessary to detect whether the light beam is scanned by the beam scanner to disperse the light source power over a scanned image field or not. If the beam scanner is malfunctioned, the light beam must be switched off to ensure safety operation. Thus, it is critical to provide a measure to detect whether the beam scanner is in normal operation condition and the light beam power is dispersed over an image field.
Therefore, it is the object of the present invention to provide a light beam scanning projection apparatus with improved image quality by detecting and compensating the phase error between the timing of modulating the light beam and the position of the scanned light beam, and to provide a safety measure for detecting both the horizontal and the vertical oscillation of the beam scanner.